1. Field of the Invention
In the present specification, the term “liquid crystal display device” means a display device that changes the polarization, scattering, or wavelength characteristics of an incident light into a liquid crystal material to display brightness and darkness thereof by means of applying a voltage to the liquid crystal.
In the present specification, the term “thresholdless liquid crystal”, “V-shaped liquid crystal”, or “liquid crystal having a chiral smectic CR phase” means a liquid crystal having no clear threshold due to remarkably occurring phase transition precursor phenomenon. The above-mentioned liquid crystal has such characteristics that, when voltages of positive polarity and negative polarity the absolute values of which are the same, are applied thereto, the tilt angles at the voltages are the same, and the transmittance is uniquely determined.
In the present specification, the term “active element” means an element that controls electric power from an external power source according to an input signal. An active element includes a thin film transistor, a field-effect transistor, and a diode.
In the present specification, the term “thin film transistor” means a semiconductor element that has a semiconductor layer, a gate electrode, a source electrode, and a drain electrode.
The present invention relates to a method of driving liquid crystal having spontaneous polarization, which can decrease hysteresis, which is specific to thresholdless liquid crystal material having no clear threshold and being capable of halftone display, thereby being capable of performing satisfactory gradation display, and simultaneously shortening the response time.
2. Description of the Related Art
A liquid crystal panel has advantages of thinness, lightness in weight, and low power consumption, and is now used in a display device such as a portable TV, a wall-hung TV, or the like, which is required to respond at high speed on the level of a dynamic image. Further, since display on a large-sized screen is possible, a liquid crystal panel can also be used as a projector panel such as of a 50-inch rear projector.
As a liquid crystal orientation mode, a TN mode with the cell gap being about 4-5 μm is commonly used since the orientation can be easily controlled in that mode. However, in the TN mode, since an orientation restraining force of an orientation film is strong, the response time is slow in halftone display which is close to a white level. Therefore, in dynamic image display at high speed, the liquid crystal material can not respond timely, which leads to flicker.
Technology for shortening the response time using nematic liquid crystal material instead of the TN mode includes a HAN mode, an OCB mode, a TN mode which shortens the response time by narrowing the cell gap to 2-3 μm to enlarge the field intensity, and a vertical orientation mode in which the liquid crystal is, since the orientation restraining force of an orientation film is week in that mode, expected to respond at the higher speed than in the case of the TN mode. However, it is difficult to realize a response time on the order of microsecond with the nematic liquid crystal.
Spontaneous polarization liquid crystal is a material to replace the nematic liquid crystal, and can respond at high speed on the order of microsecond since the interaction between spontaneous polarization and the electric field performs switching. For example, tristable antiferroelectric liquid crystal and bistable ferroelectric liquid crystal are used in a simple matrix panel. FIG. 7A illustrates the relationship between the applied voltage and the transmittance in case of tristable antiferroelectric liquid crystal, while FIG. 7B illustrates the relationship between the applied voltage and the transmittance in case of bistable ferroelectric liquid crystal. As can be seen, two levels, i.e., a white level and a black level are stable in case of tristable antiferroelectric liquid crystal and bistable ferroelectric liquid crystal. In order to perform halftone display, area gradation for displaying gray scale by adjusting the number of white pixels and the number of black pixels, time gradation for displaying gradation by adjusting the display time, or the like is used. In case the liquid crystal is used in a panel, the response of the liquid crystal is typically between the white level and the black level.
In a simple matrix panel (FIG. 8), scanning lines (X1, X2, . . . , Xn) 801 and signal lines (Y1, Y2, . . . , Yn) 802 are arranged like a matrix. When tristable antiferroelectric liquid crystal and bistable ferroelectric liquid crystal are used in a simple matrix panel, it is preferable that the hysteresis width is large. Hysteresis is the difference in transmittance due to the course of voltage. Hysteresis width 701 is defined as the difference between voltages presenting the same transmittance. For example, to drive tristable antiferroelectric liquid crystal in a simple matrix panel, it is appropriate to perform bias driving. Here, since the bias voltage is always applied, i.e., applied even when the liquid crystal panel displays the black level, in order to realize a satisfactory black level, it is desirable to use liquid crystal having clear threshold characteristics with a region where the transmittance does not change even when the bias voltage is applied thereto.
As spontaneous polarization liquid crystal capable of analog gradation, liquid crystal referred to as thresholdless liquid crystal or V-shaped liquid crystal is attracting attention (Japanese Patent Application Laid-open Nos. Hei 9-50050 and Hei 10-301091 applied by CASIO COMPUTER CO., LTD). Though thresholdless liquid crystal presents a chiral smectic CA phase in a bulk state, when it is encapsulated between substrates, differently from its ordinary state, it is orientated with a tilt with respect to the substrate main surface, and the tilt continuously changes according to the applied electric field. Since the liquid crystal presents a mixed phase over its whole thickness, the directors move smoothly with no clear threshold, which makes the hysteresis unliable to occur. This makes it possible to perform gradation display, which is impossible when tristable antiferroelectric liquid crystal and bistable ferroelectric liquid crystal are used. In addition, because of the spontaneous polarization, the interaction between the spontaneous polarization and the electric field performs switching at high speed. Domainless switching, which is impossible when tristable antiferroelectric liquid crystal and bistable ferroelectric liquid crystal are used, makes it possible to decrease domain and achieve display with a high contrast even in halftone display.
Further, thresholdless liquid crystal is said to have a SmCR* phase without a correlation of liquid crystal molecule tilt between layers (a chiral smectic CR phase) when no electric field is applied thereto. Since, when no voltage is applied, the directors of the liquid crystal are oriented at arbitrary tilt angles of cones and the tilt angle is different between the layers, the spontaneous polarization of the liquid crystal is canceled out to be zero as a whole. The average optical axis of the liquid crystal is identical with the axis of the cones (Japanese Patent Application Laid-open No. Hei 10-082985).
The hysteresis of thresholdless liquid crystal is smaller than that of the tristable antiferroelectric liquid crystal and bistable ferroelectric liquid crystal illustrated in FIGS. 7A and 7B. Since thresholdless liquid crystal has no clear threshold characteristics and the brightness increases monotonically according to the applied voltage, it is generally used in an active matrix panel. In driving where the bias voltage is always applied to a liquid crystal panel such as bias driving in a passive matrix panel, the liquid crystal is switched by the bias voltage to lower the black level.